Development of therapeutic drugs for medical and veterinary use is important in order to improve and advance treatment modalities in humans and animals. Unfortunately, many drugs that are developed and subsequently used have been found to have limited half lives in vivo due to rapid metabolism by the recipient. It would be advantageous to have a system for screening potential new drugs for susceptibility to metabolic action in patients.
A cytochrome P450 (P450) termed CYP3A4 is an important human gene that codes for an enzyme expressed in the liver, and to a lesser extent a range of other tissues. It is quantitatively the most abundant P450 in human liver. The CYP3A4 enzyme is pivotal to the metabolism of many exogenous chemicals (xenobiotics), including therapeutic drugs, as well as a range of endogenous compounds, such as steroid hormones. Changes in the level of expression of CYP3A4 can dramatically affect the rate of elimination of therapeutic drugs and thus impact on their effectiveness. A range of exogenous chemicals, including some therapeutic drugs, (hereafter termed ‘xenobiotic inducers’) are able to increase the transcriptional rate of the CYP3A4 gene and hence the formation of the CYP3A4 enzyme. The result is the elimination of drugs metabolised by CYP3A4 being greatly increased thereby diminishing their therapeutic effect.
The present inventors have obtained a DNA molecule, lying approximately 7.5 kilobases 5′ to the transcription initiation site of the human P450 CYP3A4 gene, that is responsible for the transcriptional induction of the CYP3A4 gene by xenobiotic inducers, including therapeutic drugs and is also involved in the constitutive expression of this gene. This nucleic acid molecule, called a ‘xenobiotic-responsive enhancer module’ (XREM) by the present inventors, has a number of beneficial uses.